1. Field of the Technology
The present technology relates to a color image forming method and a color image forming apparatus.
2. Description of the Related Art
In an image forming method utilizing an electrophotographic process, there is typically used a method including an image forming step of forming a toner image, a transfer step of transferring the toner image onto a recording medium, and a fixing step of fixing the toner image on the recording medium. In the image forming step, a photoreceptor drum having a surface of a photosensitive layer is used, and the surface of the photoreceptor drum is evenly charged and thereafter the surface in a charged state is exposed to light in accordance with signals corresponding to image information. An electrostatic latent image is formed by the exposure. Thereafter, by supplying a toner to the electrostatic latent image on the photoreceptor drum, a toner image is formed. In the transfer step, the toner image formed at the image forming step is transferred onto a recording medium directly or via an intermediate transfer medium. At the fixing step, the toner image on the recording medium is fixed, for example, by heating and pressing. In addition, when a color image is formed, a plurality of image forming steps of forming a toner image are provided using toners of various colors, toner images of different colors which have been formed at the respective image forming steps are transferred on the intermediate transfer medium or recording medium by sequentially overlaying one by one at a predetermined position of the intermediate transfer medium or recording medium. Consequently a multicolor toner image which is a multilayered lamination structure of the different color toner images is formed. The multicolor toner image is fixed on the recording medium to produce the color image.
In order to form the multicolor toner image, a typical image forming apparatus has a configuration in which a plurality of image forming units for carrying out the image forming step are arranged in a line from upstream to downstream of the image forming apparatus, in a rotational direction of a recording medium transporting medium or the intermediate transfer medium which is arranged so as to rotate, and a photoreceptor drum of each of the image forming units provides pressure contact with the recording medium transporting medium or the intermediate transfer medium, and the toner images on the surfaces of the photoreceptor drums are transferred onto the recording medium transported by the recording medium transporting mechanism or the intermediate transfer medium sequentially from an upstream side image forming unit. In this configuration, a time period from the transfer of the toner image formed in the image forming unit onto the recording medium or the intermediate transfer medium until fixing the transferred image on the recording medium, depends on the toner image. The toner image is also a toner aggregate whose shape is maintained by electrostatic attraction. Therefore, even though the time period from the transfer onto the intermediate transfer medium to the fixing thereto is changed in a greater or less degree, a change in shape visible to the naked eye does not occur. However, when the multicolor toner image is formed, there is provided a configuration in which the toner image transferred is further laminated with another toner image and then transferred. Therefore, a physical stress produced when the toner image is laminated and transferred facilitates dispersion of a part of the toner from the toner image, and increases a degree of the dispersion of the toner in the toner image transferred first. Accordingly, it is impossible to prevent a difference in the degree of the dispersion of the toner depending on the toner image. In particular, the toner image transferred first significantly has the above-described tendency, and has a difference in the degree of the dispersion of the toner from that of the toner image transferred last. As a result, the color image which is last formed may have insufficient color reproducibility, a color drift, a bold character, uneven image density, uneven image gloss, and the like, to decrease a transfer efficiency of the multicolor image onto the recording medium, resulting in insufficient image quality of the color image. In addition, the toner dispersed from the toner image is attached onto the surfaces of the intermediate transfer medium, the photoreceptor drums, and the like to cause an image failure such as fogging. Further, a difference in the time period from the transfer of the toner to the fixing thereof causes the degree of the dispersion of the toner in the toner image.
Meanwhile, there is proposed a color image forming method in which by using toners having respective colors, having a shape factor SF-1, which represents a degree of irregularity of surfaces of toner particles, of from 100 to 150, and containing low softening point materials such as natural waxes including a paraffin wax, and a polyolefin wax; and synthetic waxes, a plurality of toner images of different colors are formed, and intermediately transferred onto an intermediate transfer medium in a voltage applied state, to produce a multicolor toner image on the intermediate transfer medium, and the multicolor toner image is transferred onto a recording medium by a transfer section in a voltage applied status (refer to Japanese Examined Patent Publication JP-B 3066943, for example). In this color image forming method, in order to, for example, improve a transfer efficiency of the toner image onto the recording medium, and prevent fusion-bonding of the toner to the intermediate transfer medium and the photoreceptor drum, toner filming, and the like, there is provided a configuration in which a shape of the toner is defined as a numerical range of a shape factor SF-1, and a specific low softening point material is contained in the toner, and a voltage is applied to the intermediate transfer medium. However, also in an image forming apparatus for achieving this color image forming method, for example, a color drift, a bold character, uneven image density, and uneven image gloss, and the like are caused on a transferred medium. Furthermore, with respect to maintenance management such as cleaning maintenance for internal contamination of apparatuses caused by the toner dispersion, a longer time period is required than that for apparatuses according to the related art. Moreover, in JP-B 3066943, there is disclosed a technology for improving a transfer efficiency by defining a shape of the toner, but no suggestion of a technology, in particular, for preventing toner contamination of the intermediate transfer medium, caused by a change in shape of the toner image on the intermediate transfer medium.
Furthermore, there are proposed a number of technologies for defining a time constant τ of a toner in a predetermined range, in order to improve various properties of the toner. There is proposed a toner which uses a resin composition containing, for example, polyester, polyalkylene, a specific polyolefin disperser, and nonoxide polyethylene, as a binder resin, and has a time constant τ in a range of 100 msec to 350 msec (refer to Japanese Unexamined Patent Publication JP-A 2005-292468, for example). The toner disclosed in JP-A 2005-292468 is excellent in a storage property, and thus prevent mutual fusion-bonding, flocculation, and deformation of the toner when placed under heat, and excellent in a flowability, and a transfer property and a fixing property onto a recording medium, and has a tendency not to cause toner filming onto a photoreceptor drum. In a technology disclosed in JP-A 2005-292468, a time constant τ is set to in the above-described range in order to improve a charge property of the toner in a developing device, and is not set in order to prevent the toner from dispersing from a toner image on an intermediate transfer medium. In addition, in JP-B 3066943 and JP-A 2005-292468, there is not disclosed a technological idea that a plurality of toners of different time constants τ in addition to different colors are used upon forming a multicolor toner image.